Make-alive electrode



May 9, 1939.v J. SLEPI'AN MAKE-ALIVE ELECTRODE Filed March 31, 1958 INVENTOR ATTORNEY a ose v77 5 [67 1072.

WITNESSES:

g. 74 aw Patented May 9, 1939 UNITED STATES PATENT @FFITCE MAKE-ALIVE ELECTRODE sylvania Application March 31, 1938, Serial No. 199,146

14 Claims.

My invention relates to a make-alive electrode for an electric arc discharge device and more particularly to a carbonaceous electrode and the method of constructing the same.

In the operation of electric arc discharge devices, it has been found advantageous to employ make-alive electrodes of some high resistance material such as boron carbide, silicon carbide and other materials known under the trade names of Carborundum, Globar or Silit to initiate an are at the beginning of each conducting period in the rectifier. Make-alive electrodes generally used are in the form of a solid rod made up of one of the above mentioned materials. The

I dimensions of these electrodes, particularly the diameter, must be such that they will be strong enough to withstand jarring upon being assembled and also capable of withstanding the forces exerted by the mercury vapor as well as the motion of the mercury while under the influence of the cathode spots. Consequently, makealive electrodes have been constructed of sufficient diameter, due to the brittleness of the material they are composed of, to be mechanically strong.

In passing an exciting current through such electrodes, it was found that a substantial part of the current passes through the, electrode with no useful work accomplished. According to the teachings of the art, one of the requirements for the starting of an arc cathode at the junction of the electrode and the mercury cathode is the maintenance of a sufiicient current concentration at that point. Engineers have been led to shape and design make-alive electrodes in various forms with the ultimate objective of reducing the useless current to as low a point as possible.

According to my invention, I have diverged from the ordinary design of a make-alive electrode made up of a solid rod of some suitable high resistance material for I have found it practical to use a suitable rod of ceramic material and cover the exterior surface thereof with a thin layer of carbon. It is immaterial in what shape or form the exterior or the active portion of my electrode may be made for it is to be understood that the essential purpose of my invention is to narrow the current path through the starting electrode to a minimum. In one form of my electrode, I coat a non-conducting rod with some carbonaceous material and carbonize it by heating the coated rod in-suitable non-oxidizing atmosphere for several hours. Several applications of the coating material is placed on the rod and carbonized in the usual manner un-' til the desired thickness is obtained.

According to my experiments, I have found that if the thickness of the layer is in the order of one millimeter or less the electrode functions Well. In another form of my electrode, a plurality of resistance plates, preferably of carbon, are mounted on a suitable non-conducting rod and held under pressure. The resistance along the stack is in the neighborhood of sev-' eral ohms per centimeter length. On the other hand, the resistivity in the radial direction is considerably lower. In such case, a high current concentration is obtained where the resistance plate is in contact with the mercury, this being an essential requirement for the starting of an arc.

'It is, therefore, an object of my invention to provide a make-alive electrode for an electric arc discharge device having a carbon surface on a non-conducting base.

Another object of my invention is to provide a make-alive electrode having a high current concentration at the junction of the electrode material and the mercury cathode.

Still further, it is an object of my invention to provide an improved type of make-alive electrodefor an electric discharge device which is reliable in operation and sturdy in construction. Other objects and advantages of my invention-wil1 be apparent from the following detailed construction taken in conjunction with the accompanying drawing, in which:

Figure 1 is a diagrammatic illustration of a vapor electric converter with which my improved make-alive electrode may be used,

Fig. 2 represents in section an improved makealive' electrode according to my invention, and;

Figs. 3 and 4 illustrate possible modifications thereof.

For a more complete understanding of the operation of the converter employing my improved starting electrode with which my invention is concerned, I have illustrated in Fig. l a typical arrangement of a vapor electric converter Shaving an anode 5, a mercury pool cathode 1 and a starting electrode 9 of the immersion-ignitor type. The cathode l and the anode 5 are externally'connected with a load H and some suitable current supply circuit H. In order to initiate a discharge between the anode 5 and the cathode l, itis essential that the anode 5 be maintained positive with respect to the cathode 1. Likewise, it is essential to provide an immersion-type starting electrode 9 having. its tip portion projecting into the mercury pool cathode l. The starting electrode 9 is also connected externally to some suitable control device [5 for supplying exciting current which is adapted to cause conduction of current from the starting electrode 9 into the mercury pool cathode I at predetermined instances of time with respect to the polarity of the main anode potential.

Referring more particularly to Fig. 2, I show a make-alive electrode 9 having a non-conducting stem portion I! of suitable ceramic material preferably quartz having its exterior surface covered with some carbonaceous material I9. I have observed from experiments that such an electrode 9 possesses desirable electrical and mechanical characteristics to warrant its use as a makealive electrode 9. It was also observed that the amount of exciting current required to initiate an arc discharge depends to some extent on the diameter of the electrode 9 in contact with the mercury cathode I. In other words, the smaller the diameter of the electrode 9 the narrower the current path for the exciting current and consequently the greater the current concentration along the active portion of the electrode 9.

In constructing a make-alive electrode 9 according to my invention, I place a layer of carbonaceous material l9, such as pitch, on a nonconducting rod l1, preferably quartz, and place the rod l l in a crucible where it is heated to approximately 1000 C. for several hours in some non-oxidizing atmosphere. Repeated applications of carbonaceous material may be made and the rod I'l carbonized again until the thickness of about one-tenth of a millimeter is obtained. A suitable metal cap 2| is mounted over a portion of the carbonized rod IT. The resistivity of such a layer of carbon I9 is much higher than in bulk so that a resistivity of about ohms per cubic centimeter may be obtained which is approximately 100 fold greater than the resistivity of mercury. Of course, the most desirable resistance to use is governed by the current and voltage limits that can be tolerated. Generally, the resistivity of the active portion IQ of the makealive electrode 9 should be such as to maintain a potential gradient of the order of 100 Volts per centimeter along the electrode 9 without passing excessive current through it.

According to Fig. 3 of my invention, the active portion I9 of the make-alive electrode 9 takes a difierent form for I have previously mentioned that it is immaterial in what manner the exterior covering of the active substance i9 is placed on the non-conducting rod ll. In this modification, I take a multiplicity of annular carbon plates 23 having a thickness of about one millimeter and stack them one upon another on a non-conducting rod ll having an enlargement i8 at its tip end to prevent the plates 23 from falling off. I maintain the plates 23 under moderate pressure so that a high resistance electrical contact is made between successive plates 23. One particular starting electrode 9 I have constructed has an internal diameter of one-eighth of an inch and an external diameter of one-quarter of an inch. It naturally follows, that the resistance along the stack is in the order of several ohms per centimeter length. On the other hand, the resistivity in the radial direction is proportionately much lower.

Upon passage of exciting current through such a make-alive electrode 9 the current flows through each plate 23 in succession until it reaches the first plate 23 in contact with the mercury cathode 1 where the current immediately flows in a radial direction, thus concentrating at the point where the resistance plate 23 touches the mercury cathode 1. By such a construction, I maintain the active or useful current to the total current large, a condition favorable for a quick formation of an arc.

In accordance with Fig. 4 of my invention, the make-alive electrode 9 differs from the one shown in Fig. 3 only in one respect. In this modification, the resistance plates 23 are of two diameters and are so assembled on the non-conducting rod IT as to secure an alternate formation of the larger and smaller size plates 23. Experiments reveal that mercury will not enter slots of the order of one millimeter at ordinary depths of an electrode in mercury due to the high surface tension of the latter. Accordingly, the electrode construction I provide utilizes this principle for in effect the mercury cathode 9 touches only every other annular plate 23, thereby obtaining an improved action insofar as the voltage gradient is concerned.

I have described my invention as being particularly adapted to mercury-pool converters, but it will be obvious that its use may be equally applicable to other liquid or solid conductors such as gallium or solidified tin which are suitable for use as cathode material.

While I have illustrated and described a specific embodiment of my invention, it will be apparent that changes and modifications can be made therein without departing from the true spirit of my invention or the scope of the appended claims.

I claim as my invention:

1. A make-alive electrode for an electric arc discharge device comprising a non-conducting stem portion, a covering of semi-conducting material on said stem and a conducting portion mounted over a portion of said stem.

2. A make-alive electrode for an electric arc discharge device comprising a non-conducting stem portion and an active portion of carbonaceous material covering the exterior surface of said stem.

3. A make-alive electrode for an electric arc discharge device comprising a stem portion of ceramic material with its exterior surface carbonized and a metallic portion electrically in contact with said carbonized portion.

4. A make-alive electrode for a vapor electric converter comprising a. non-conducting stem portion, a plurality of resistance plates mounted on said stem portion and a conducting cap fitted over a portion of said stem, said cap maintained in contact with at least one of said plates.

5. A make-alive electrode for a vapor electric converter comprising a stem of non-conducting material, a plurality of annular resistance discs mounted over said stem, said discs being maintained in pressure engagement with each other and a conducting cap contacting some of said discs.

6. The method of producing a make-alive electrode which comprises heating a carbon-coated ceramic rod in a non-oxidizing atmosphere.

7. The method of producing a make-alive electrode comprising a refractory stem portion, painting said stem with carbonaceous material, heating said electrode in a non-oxidizing atmosphere and repeating applications of said carbonaceous material and reheating until a predetermined thickness is obtained.

8. A make-alive electrode assembly for an electric arc discharge device comprising a stem portion of insulating material, an enlargement on the tip portion of said stem, a plurality of resistance plates arranged in cylindrical formation on said stem substantially to said enlargement and a metallic cap in contact with at least some of said resistance plates.

9. A make-alive electrode for an electric arc discharge device comprising a rod of non-conducting material, a plurality of resistance plates having at least two dissimilar diameters mounted on said rod, said dissimilar sized plates alternately arranged on said rod.

10. A make-alive electrode for an electric arc discharge device comprising a quartz stem, a multiplicity of flat annuli of resistance material mounted on a portion of said stem, said annuli of resistance material held pressed together and a metallic cap mounted over the remaining portion of said stem.

11. In combination with an electric are discharge device having an anode, a mercury pool cathode and a make-alive electrode comprising a rod-like body of ceramic material, a plurality of resistance plates mounted on a portion of said rod and a metallic connector in tight fitting engagement with the remaining portion of said rod, said make-alive electrode dipping into said cathode in such manner that the latter is in contact with several of said resistance plates.

12. In combination with an electric are discharge device having an anode, a mercury pool cathode and a make-alive electrode comprising a rod-like body of ceramic material, a plurality of resistance plates of at least two dissimilar diameters mounted on said rod, said dissimilar sized plates being alternately arranged on a portion of said rod, a metallic cap on theremaining portion of said rod, said make-alive electrode dipping into said cathode whereby the latter is in contact With several of said resistance plates.

13. A starting electrode for a vapor electric converter comprising an insulating base member, an active member of semi-conducting material disposed on the surface of said base member and a conducting cap member mounted over a portion of said active member.

14. A starting electrode for a vapor electric converter comprising a core of insulating material and a semi-conducting member enclosing said core.

JOSEPH SLEPIAN. 

